The invention generally is directed to a method for compensating alternating electrical fields present at the front surface of a cathode picture tube. Additionally, the invention generally is directed to an apparatus that is suitable for carrying out the method. More specifically, the invention is directed to a method and apparatus for compensating the alternating electrical field at the front surface of a cathode tube produced by a high-voltage signal generator that supplies a high-voltage signal to the picture tube via an EHT high-voltage terminal.
Many countries have safety rules and recommendations for users of cathode ray picture tubes that, for example, are employed in television equipment and video equipment, or as data terminal equipment and monitors in data processing equipment. These rules generally provide that the alternating electrical fields outside of the apparatus employing the picture tubes not exceed a defined value.
To that end, Swedish and other authorities have specified that at a distance of 30 centimeters from the front surface of a cathode ray picture tube, the voltage of the alternating electric field not exceed 1 volt per meter in the frequency range of 2 kHz to 400 kHz. Additionally, these authorities have specified that the voltage of the electric field at this distance not exceed 10 volts per meter in the frequency range of 5 kHz through 2 kHz.
Without the use of special structures or arrangements to compensate for the electrical fields present at the front surface of a picture tube, the strengths of the alternating electric field of concern in these frequency ranges have been known to amount to several tens of volts per meter.
In accordance with prior methods, it is possible to weaken the alternating electrical field by placing a glass plate in front of the front surface of the picture tube. The glass plate generally is coated with a low-impedance layer by means of which charges occurring due to the alternating field are drained or flow off, so that the effect of these charges is largely cancelled. However, the image quality of the picture is degraded by the glass plate. Moreover, the glass plate presents a not inconsiderable additional cost.
It is also possible to attenuate the alternating voltage signal of the picture tube that is responsible for generating the alternating electrical field in front of the picture tube. The high-frequency portions of this alternating voltage derive from the deflection frequency and its harmonics, particularly from the capacitive coupling in the rectifier of the high-voltage generator (also referred to as an EHT generator), and from the capacitive coupling of the deflection coils. The low-frequency portion of this alternating voltage results from current fluctuations in the picture tube and by the internal impedance of the EHT generator. Although these frequencies can be decoupled with a capacitor, this generally is impractical when the strength of the alternating electrical field is to be kept below the required lower limit of 1 volt per meter given a distance of 30 centimeters in front of the picture tube, because a large external capacitor would be required for this purpose.